From Bots, #StrongerIn, and #Brexit: Computational Propaganda during the UK-EU Referendum by Philip N. Howard and Bence Kollanyi:
The most active users — the accounts that tweeted 100 or more times with a related hashtag during the week — generated 32 percent of all Twitter traffic about Brexit. That volume is significant, considering that this number of posts was generated by fewer than 2,000 users in a collection of more than 300,000 users. In other words, less than 1 percent of the accounts generate almost a third of all the content. However, not all of these users or even the majority of them are bots. Anecdotally, it is difficult for human users to maintain this rapid pace of Twitter activity without some level of account automation.
H/T to Gregory Walton.